Multifilament textile yarns with hollow section, method for making same, and textile surfaces obtained from said yarns

ABSTRACT

The invention concerns a multifilament textile yarn whereof the filaments or staples have a hollow section, a method for making said hollow yarn, and textile surfaces obtained from said yarns. More particularly, it concerns a method for making multifilament yarns comprising hollow filaments obtained by melt-drawing of a polyamide composition having a yarn count less than 10 dtex for each staple and whereof the staples with hollow section have a central hollow surface representing at least 5% of the total surface in said staple transfer section. The invention is characterized in that the yarn has an USTER coefficient (U %) less than 3% and a number of staple with arc-shaped section less than 30% of the total number of staples with hollow section. The flat yarns, twisted and textured are used in particular for mixing textile surfaces, such as woven or knitted fabric.

The present invention relates to a multifilament textile yarn, thefilaments of which have a hollow cross section, to a process formanufacturing this hollow yarn and to the textile surfaces obtained withsuch a yarn.

It relates more particularly to the manufacture of multifilament yarnscomprising hollow filaments obtained by melt spinning a polyamidecomposition.

The use of hollow textile yarn in the production of textile surfaces hasbeen sought for a very long time for producing lighter and thermallymore insulating garments. However, the manufacture of filaments whichinclude a large hollow volume and exhibit good regularity has proved tobe very difficult, especially at spinning speeds and filament countswhich are compatible with industrial manufacture and are suitableespecially for textile applications.

A process was proposed in 1956 for manufacturing a hollow polyamideyarn. This process is described in Patent GB 843,179. It consists inspinning the polyamide in a spinneret which includes spinneret holes inthe form of just one circular arc, the ends of the said arc beingseparated by a distance of between 0.05 and 0.3 mm.

That patent describes the manufacture of nylon-6 filaments obtained at aspinning rate of 700 m/min. The filaments are then subjected to adrawing step in order to obtain the desired count.

The spinning rate is very low compared with the rates of currentindustrial processes which are often greater than 3000 m/min.Furthermore, in the current processes the drawing step is oftenintegrated into the spinning, without intermediate reworking of thefilaments.

A process for obtaining polyamide filaments with a hollow cross sectionby a high-speed (greater than 1500 m/min.) spinning process has alsobeen proposed in Patent Application WO 95/25188. However, this processrequires the spinning of a polyamide with a high viscosity (V_(R)greater than 50 and preferably greater than 60). Such a viscosity ishigh compared with that of the polyamides employed for the manufactureof conventional textile yarns, which is generally less than 50. Thatdocument specifies that the production of hollow filaments according tothe process described is impossible with a polyamide of V_(R) less than50. Thus, the process described in that document requires an additionalstep in order to increase the viscosity of the polyamide, for example bypostcondensation or by using catalysts. The spinneret used in thatdocument has spinneret holes consisting of several (at least 2) circulararcs, the ends of which are a small length apart.

One of the objects of the present invention is to propose amultifilament textile yarn, the filaments of which have a hollow crosssection, the yarn being obtained by a spinning or spinning-drawingprocess which has the advantages of the current industrial processes forspinning filaments with a solid cross section, namely productivity andregularity of the properties of the yarn, by using a polyamide of V_(R)similar to that of the polyamides used in the manufacture of textileyarns.

For this purpose, the invention proposes a multifilament polyamidetextile yarn having a filament count of less than 10 dtex and thefilaments of which have a hollow cross section comprising a centralempty area representing at least 5% of the total area of the crosssection of the said filament, characterized in that the yarn has anUster coefficient (U %) of less than 3% and the number of filamentshaving a cross section in the form of a circular arc is less than 30% ofthe total number of filaments having a hollow cross section.

The term “filament having a hollow cross section” should be understoodto mean all the filaments obtained from a spinneret hole in the form ofa circular arc, as will be described below. In other words, the hollowfilaments are those whose cross section is in the form of an open orclosed ring.

According to the invention, the multifilament yarn has a U % of lessthan 2%, preferably less than 1%.

Furthermore, the number of filaments in the form of a circular arc isadvantageously less than 20%, preferably less than 10%, of the totalnumber of filaments having a hollow cross section.

The term Up or Uster coefficient is representative of the regularity ofa continuous yarn. Thus, this coefficient is determined by measuring thevariations in the count of a continuous yarn on an Uster machine(Zellweger).

In order to carry out this measurement, the yarn maintained under acertain tension runs at a constant speed between the plates of acapacitor, the variation in the dielectric element formed by the yarn,due to variation in mass of the yarn, causes a variation in thecapacitance of the capacitor, which is translated into the irregularityin the yarn count.

A graph showing the irregularities in the count or variation in massalong the yarn is plotted, the Uster coefficient (U %) representing themean variation in mass, expressed as % of the mean mass.

According to another preferred characteristic of the invention, thepercentage of empty area in the cross section of the filaments isgreater than 10%, preferably greater than 15%, of the cross section ofeach filament. Advantageously, this value is between 15 and 50%.

According to another characteristic of the invention, the multifilamentyarn is obtained by spinning, using a spinneret, a polyamide-basedcomposition having a relative viscosity V_(R) of greater than 40 andpreferably between 40 and 55, advantageously between 40 and 50.

The relative viscosity V_(R) of the polyamide is determined on an 8.4%polymer solution in formic acid.

The polyamide suitable for the invention is chosen from the groupadvantageously comprising nylon-6, nylon-6,6, polyamide blendscomprising at least 80% by weight of nylon-6,6 or nylon-6, andcopolyamides comprising at least 80% by number of nylon-6,6 or nylon-6units.

The yarn of the invention exhibits excellent regularity and has no openfilaments or filaments with a cross section in the form of a circulararc, that is to say which have not coalesced at the spinneret exit, andavoids the problems of breakage and buckling in the downstreamtreatments of the yarn, such as, for example, texturizing, dyeing,weaving or knitting, as well as irregularities on the textile surfaces.

The subject of the invention is also a process for manufacturing amultifilament yarn possessing the characteristics indicated above.

According to the invention, the process for manufacturing amultifilament polyamide yarn comprising filaments of hollow crosssection consists in extruding a polyamide composition through at leastone spinneret comprising several spinneret holes in order to formfilaments, in making at least some of the said filaments converge at apoint remote from the spinneret in order to form the said multifilamentyarn and then to wind up the said yarn on a reel, characterized in thateach spinneret hole consists of a slot in the form of a circular arc,the ends of which are separated by a length at most equal to 5 times themean width of the said slot emerging at the exit face of the spinneret,the filaments being cooled on leaving the spinneret and the yarn beingwound up at a rate of greater than 3000 m/min., preferably greater than3500 m/min.

According to another characteristic of the invention, the width of theslot forming a spinneret hole on the exit surface of the spinneret isadvantageously between 0.05 mm and 0.12 mm.

According to a novel characteristic of the invention, the ends of thecircular arc forming each spinneret hole are symmetrical over at least5% of the total length of the said circular arc, preferably Over atleast 15% of this total length.

The spinnerets are produced using the standard techniques, such aselectrical discharge machining.

Advantageously, the density of holes on a spinneret is between 0.25holes/cm² and 3 holes/cm².

The yarn leaving the spinneret is cooled. The distance between the startof the cooling and the exit of the spinneret is as short as possibletechnologically. Thus, immediate cooling at the spinneret exit may bedifficult to employ since it may cause polymer to be deposited on thesurface of the spinneret, which would have the consequence of breakingthe filaments.

According to the invention, the cooling is obtained by a coolant chosenfrom the group comprising air, a non-oxidizing gas or an inert gas, suchas nitrogen. Preferably, this coolant is not saturated with watervapour.

In a preferred embodiment of the invention, the coolant is air or aninert gas.

The processes for treating the yarns thus formed are those normally usedfor the manufacture of polyamide textile yarns. Thus, sizingcompositions for improving the surface lubrication and for altering thehydrophilic or hydrophobic properties of the surface, may be applied.

Generally, these yarns are subjected to a cold-drawing or hot-drawingstep, possibly in the presence of steam. This drawing is generallyincorporated into the spinning process, that is to say carried outbefore winding up the yarn. However, without departing from the scope ofthe invention, the yarns may also be drawn in a drawing processindependent of the spinning, by reworking the wound-up yarn.

It is also possible to intermingle the filaments in order to improve thecohesion of the yarn. This intermingling is often used before thedrawing step, but it may be done during or after the latter.

Next, the polyamide yarn may be relaxed before being wound up on a reel.

The multifilament yarns of the invention may be subjected to a crimpingstep or a texturizing step, depending on the field of application.

Thus, these yarns may be used as flat yarn or as twisted or texturedyarn, in weaving processes as warp yarn or as weft yarn, for example.

The yarns of the invention are also used for the manufacture of knittedsurfaces. Such textile surfaces woven or knitted with at least one yarnof the invention are also subjects of the invention.

Further details and advantages of the invention will become more clearlyapparent in the light of the detailed description of at least oneembodiment, given purely by way of indication, with reference to theappended drawings in which:

FIG. 1 shows a schematic view of the shape of a spinneret hole accordingto the present invention; and

FIG. 2 is a photographic sectional view of a multifilament yarnaccording to the invention.

EXAMPLE 1

The yarns according to the invention are obtained by spinning acomposition comprising a PA-6,6 polyamide of relative viscosity V_(R)equal to 49 and a flatting agent consisting of titanium dioxide at aconcentration of 0.3% by weight.

This composition is fed into a conventional spinning device comprising aspinneret suitable for the spinning of hollow yarn.

Thus, with reference to FIG. 1, the spinneret comprises 40 spinneretholes (hole density equal to 0.8 holes/cm²) having a profile 1 in theform of a circular arc of external diameter equal to 1.5 mm. The ends 1a, 1 b of the circular arc are separated by a distance equal to 0.15 mm.The width 1 of the slot is regular in the example illustrated and isequal to 0.08 mm. However, this width may differ over the length of theslot without thereby departing from the scope of the invention, whilemeeting the symmetry characteristic of the end parts of the slot asdefined above.

The spinning conditions are the following:

pressure: 250 bar

temperature: 280° C.

polymer flow rate: 2.3 g/min/hole.

The filament bundle is cooled as it leaves the spinneret by a stream ofcoolant fed in a direction approximately perpendicular to the directionin which the bundle runs. This feed is placed at a distance of 30 mmfrom the exit surface of the spinneret. In the example illustrated, thecoolant is air at a temperature of 22° C. Of course, the stream ofcoolant is fed at a rate which does not disturb the running of thefilament bundle.

Next, the filaments are made to converge on a point 130 cm away from thespinneret exit. A sizing composition is deposited on the yarn thusformed, using conventional processes and devices.

Next, the yarn is wound up on a reel at a wind-up rate equal to 4500m/min.

The yarn obtained has a total count of 165 dtex and comprises 40filaments (165F40 yarn count).

FIG. 2, depicting a cross-sectional view of the yarn obtained, shows thehollow cross section in the form of a ring of filaments. The percentagevoid content is greater than 15%. The number of filaments having a crosssection in the form of a circular arc, that is to say filaments whoseends have not coalesced at the spinneret exit, is equal to zero in theexample illustrated.

The characteristics of the yarn obtained are as follows:

Uster coefficient (U %): 0.9 (value determined by integration)

elongation at break: 45%

tenacity: 35 cN/tex

S_(BW) (shrinkage in boiling water): 10%

Cohesion factor (determined by the manual hook method): 15 knots/m.

A fabric was woven with yarns obtained by the process described aboveand having a 165F40 count. The fabric was woven on a rapier loom andsubjected to a calendering operation.

An identical fabric was woven with a conventional yarn having a roundand solid cross section. The weft yarn and warp yarn count is 235F34.

The properties of these two fabrics are given in the table below.

TABLE I Fabric with Fabric with yarns of yarns of the solid crossCharacteristics invention section Grammage (g/m²) 95 136 Air Pressure:79.8 72.9 permeability 100 Pa (l/m²/s) Pressure: 114 111 160 Pa ThermalThickness 0.1872 0.1456 properties (mm) ΔT = 10 K Thermal 0.006380.00606 G = 200 g resistance coefficient (Km²/W) Conductivity 0.028340.0241 coefficient (W/m²K)

These tests were carried out on the ALAMBETTA T675 apparatus sold byVolta S.p.A.

These tests show that the fabric woven with the yarns of the inventionhas an equivalent air permeability for a grammage which is 30% less thanthat of a fabric woven with conventional yarns. Furthermore, theconductivity coefficient is markedly less for the fabric woven with theyarns of the invention, thus making it possible to produce moreinsulating garments for a lower or equivalent weight.

EXAMPLE 2

The yarns according to the invention are obtained by spinning, in anintegrated spinning/drawing process, a composition comprising a PA-6polyamide of relative viscosity V_(R) equal to 50 and a flatting agentconsisting of titanium dioxide at a concentration of 0.4% by weight.

This composition is fed into a conventional spinning device comprising aspinneret suitable for the spinning of hollow yarn identical to thatused in Example 1, but having 20 holes per spinneret.

The spinning conditions are as follows:

pressure: 220 bar

temperature: 272° C.

polymer flow rate: 1.75 g/min/hole.

The filament bundle is cooled as it leaves the spinneret by a stream ofa coolant fed in a direction approximately perpendicular to thedirection in which the bundle runs. This feed is placed at a distance of40 mm from the exit surface of the spinneret. In the exampleillustrated, the coolant is air at a temperature of 20° C. Of course,the stream of coolant is fed at a rate which does not disturb therunning of the filament bundle.

Next, the filaments are made to converge on a point 130 cm away from thespinneret exit. A sizing composition is deposited on the yarn thusformed, using conventional processes and devices.

Next, the yarn is wound up on a reel at a wind-up rate equal to 4500m/min.

The yarn obtained has a total count of 78 dtex and comprises 20filaments (78F20 yarn count).

The yarn obtained has a percentage void content greater than 15%. Thenumber of filaments having a cross section in the form of a circulararc, that is to say filaments whose ends have not coalesced at the exitof the spinneret, is equal to zero.

The characteristics of the yarn obtained are as follows:

Uster coefficient (U %): 1 (value determined by integration)

elongation at break: 35%

tenacity: 38 cN/tex

cohesion factor: 10 knots/m (determined by the manual hook method).

What is claimed is:
 1. Multifilament polyamide textile yarn having afilament count of less than 10 dtex and the filaments of which have ahollow cross section having a central empty area representing at least5% of the total area of the cross section of the said filament, whereinthe yarn has an Uster coefficient (U %) of less than 3% and the numberof filaments having a cross section in the form of a circular arc isless than 30% of the total number of filaments having a hollow crosssection.
 2. Yarn according to claim 1, wherein having it has a U % ofless than 2%.
 3. Yarn according to claim 2, wherein having a U % of lessthan 1%.
 4. Yarn according to claim 1, wherein the number of filamentshaving an open cross section or a cross section in the form of acircular arc is less than 20%, preferably less with respect to thenumber of filaments having a hollow cross section in the yarn.
 5. Yarnaccording to claim 1, wherein the empty area of a hollow cross sectionof a filament represents at least 10% of the total cross section of saidfilament.
 6. Yarn according to claim 1, wherein the polyamide iscomprises nylon-6, nylon-6,6, polyamide blends comprising at least 80%by weight of nylon-6,6 or nylon-6, and copolyamides comprising at least80% by number of nylon-6,6 or nylon-6 units.
 7. Yarn according to claim1, wherein the polyamide has a relative viscosity V_(R) of between 40and
 55. 8. Yarn according to claim 7, wherein the relative viscosity isbetween 40 and
 50. 9. Process for manufacturing a multifilamentpolyamide yarn according to claim 1 comprising filaments of hollow crosssection, comprising extruding a polyamide composition through at leastone spinneret comprising several spinneret holes in order to formfilaments, making at least some of said filaments converge at a pointremote from the spinneret in order to form said multifilament yarn andthen winding said yarn on a reel, wherein each spinneret hole comprisesa slot in the form of a circular arc, the ends of which are separated bya length at most equal to 5 times the mean the said slot emerging at theexit face of the spinneret, the filaments being cooled on leaving thespinneret and the yarn being wound up at a rate of greater than 3000m/min.
 10. Process according to claim 9, wherein the hollow area of thecross section of each filament is equal to at least 5% of the total areaof the cross section of the filament.
 11. Process according to claim 10,wherein the amount of hollow area is equal to at least 10% of the areaof the cross section of the filament.
 12. Process according to claim 9,wherein the mean width of the slot is between 0.05 mm and 0.12 mm. 13.Process according to claim 9, wherein in that the coolant for coolingthe filaments beneath the spinneret comprises air and/or the an inertgas.
 14. Process according to claim 9, wherein the yarn comprises atmost 30% by number of filaments having a cross section in the form of acircular arc compared with the number of filaments having a closedhollow cross section.
 15. Process according to claim 9, wherein the yarnhas an Uster coefficient of less than 3%.
 16. Process according to claim9, wherein the ends of the circular arc forming each spinneret hole aresymmetrical over at least 5% of length of the said circular arc. 17.Process according to claim 16, wherein the symmetrical ends of eachcircular arc have a length equal to at least 15% of the total length ofa circular arc.
 18. Process according to claim 9, wherein the polyamidehas a relative viscosity of between 40 and
 55. 19. Process according toclaim 18, wherein the polyamide has a relative viscosity of between 40and
 50. 20. Process according to claim 9, wherein the polyamidecomprises nylon-6, nylon-6,6, polyamide blends comprising at least 80%by weight of nylon-6,6 or nylon-6, and/or copolyamides comprising atleast 80% by number of nylon-6,6 or nylon-6 units.
 21. Process accordingto claim 9, wherein the yarn is subjected to a drawing operation beforeit is wound up.
 22. Flat, textured or twisted yarn obtained from amultifilament yarn according to claim
 1. 23. Woven or knitted textilesurface obtained from at least one yarn according to claim 1.